1. Field of the Invention
The present invention relates to a process for purifying styrene based polymers.
2. Description of the Prior Art
By comparison with homopolymers of styrene, copolymers containing styrene offer many advantages, in particular, improved mechanical properties, lower thermal deformation and better ageing. The most generally used styrene based copolymer are:
San resins resulting from the copolymerization of styrene and acrylonitrile, PA1 Hips resins prepared by polymerizing styrene in the presence of an elastomer, typically a polybutadiene rubber, PA1 Abs resins produced from styrene, acrylonitrile and an elastomer. PA1 benzenesulfonylhydrazide : 104.degree. C. for pure product PA1 1,3-benzenedisulfonylhydrazide: 163.degree. C. for pure product 145.degree. C. for commercial product PA1 p-tolylsulfonylhydrazide : 110.degree. C. PA1 4,4-oxybis(benzenesulfonylhydrazide): 164.degree. C. for pure product 130.degree. C. for commercial product PA1 3,3-sulfobis(benzenesulfonylhydrazide): 153.degree.-158.degree. C.
For preparing these resins, conventional polymerization processes such as mass, suspension or emulsion polymerization may be used. In commercial practice, however, a two-step process is most often used. Such a process comprises a mass prepolymerization at a temperature between approximately 75.degree. and 125.degree. C., followed by a suspension polymerization, at a temperature in the range of approximately 75.degree. to 150.degree. C.
The styrene based polymers formed are thermoplastic materials and they can be molded by any conventional method. They are used, in particular, for the manufacture of various types of containers for liquid and solid products.
It has been found that when using any one of the above polymerization processes, residual monomer remains in the resin. Until now it has not been possible to carry out the polymerization on an industrial scale to the very end in order to avoid the presence of residual monomer. For instance, HIPS, SAN and ABS resins generally contain about 0.1 to 0.5% by weight of residual styrene.
For some uses, namely when these resins are used as materials for packaging of edible products, this content is too high and residual monomer migrates slowly from the container to the contents of the container. The amount of monomer which will migrate depends on the amount of residual monomer in the resin.